Catalyst systems and their components that are suitable for preparing a polyolefin are generally known. One type of such catalysts are generally referred to as Ziegler-Natta catalysts. The term “Ziegler-Natta” is known in the art and it typically refers to catalyst systems comprising a transition metal-containing solid catalyst compound (also typically referred to as a procatalyst); an organometallic compound (also typically referred to as a co-catalyst) and optionally one or more electron donor compounds (e.g. external electron donors).
The transition metal-containing solid catalyst compound comprises a transition metal halide (e.g. titanium halide, chromium halide, hafnium halide, zirconium halide or vanadium halide) supported on a metal or metalloid compound (e.g. a magnesium compound or a silica compound). An overview of such catalyst types is for example given by T. Pullukat and R. Hoff in Catal. Rev.—Sci. Eng. 41, vol. 3 and 4, 389-438, 1999. The preparation of such a procatalyst is for example disclosed in WO96/32427 A1.
One of the functions of an external donor compound is to affect the stereoselectivity of the catalyst system in polymerization of olefins having three or more carbon atoms. Therefore it may be also referred to as a selectivity control agent.
The use of silicon compounds as external donors is known in the prior art as being used as external electron donors in Ziegler-Natta catalyst systems for polymerization of olefins. The art presently recognizes a finite set of compounds suitable for use as external donors.
Documents EP1538167 and EP1783145 disclose a Ziegler-Natta catalyst type comprising an organo-silicon compound as external donor that is represented by formula Si(ORc)3(NRdRe), wherein Rc is a hydrocarbon group having 1 to 6 carbon atoms, Rd is a hydrocarbon group having 1 to 12 carbon atoms or hydrogen atom, and Re is a hydrocarbon group having 1 to 12 carbon atoms used as an external electron donor.
Typical external donors known in the art (for instance as disclosed in documents WO2006/056338A1, EP1838741B1, U.S. Pat. No. 6,395,670B1, EP398698A1, WO96/32426A) are organosilicon compounds having general formula Si(ORa)4-nRbn, wherein n can be from 0 up to 2, and each Ra and Rb, independently, represents an alkyl or aryl group, optionally containing one or more hetero atoms for instance O, N, S or P, with, for instance, 1-20 carbon atoms; such as n-propyl trimethoxysilane (nPTMS), diisobutyl dimethoxysilane (DiBDMS), t-butyl isopropyl dimethyxysilane (tBiPDMS), cyclohexyl methyldimethoxysilane (CHMDMS), dicyclopentyl dimethoxysilane (DCPDMS), di(iso-propyl) dimethoxysilane (DiPDMS).
EP 1 197 497 relates to a process for producing PP and/or random copolymers of propylene type with lesser formation of lump. US2002/007024 relates to a process for producing polyethylene with a Ziegler-Natta type catalyst and ether type external electron donors. U.S. Pat. No. 4,921,919 relates to a process for vapor-phase polymerization of monomers for minimizing the formation of polymer agglomerates or lumps. Chan et al (“Syntheses and ultraviolet spectra of N-organosilyl ketimines” J. Organometal. Chem. 1967, vol. 9 no. 2, pp 231-250) relates to the synthesis of N-organosilyl ketimines. U.S. Pat. No. 3,622,529 relates to a stable composition comprising silanol chain-stopped polydiorganosiloxanes. CA 957 695 relates to the synthesis of imidatosilanes from imidate and a chlorosilane.
However, by using such external electron donors known in the prior art, high formation of lumps in the powder polymer products within the reactor vessel and in the powder polymer product might occur under certain circumstances. Polymer chunks or lumps not only hamper production, reducing reaction rates and production rates but also induce a greater amount of risks, such as injuries and fire while removing polymer chunks using normal maintenance practices. In addition, lumps in the product result in a non-uniform size product and lumps inside the reactor vessel can result in stoppage of the process requiring cleaning of the reactor vessel before the process can be continued. This can be quite costly and time consuming.
There is, therefore, an on-going need in industry for catalysts showing better or varied performance in polymerization of olefins without hampering production of polyolefins.
It is thus an object of the invention to provide an improved catalyst system having high hydrogen and ethylene response that allows obtaining of a polyolefin, preferably a propylene-based polymer with high isotacticity, while minimizing the formation of polymer agglomerates and lumps in the reactor for making the polyolefin.